Electronic device and electronic system

ABSTRACT

One object of an embodiment is to provide a novel electronic device which is configured so that a user can read data regardless of a location, input data by directly touching a keyboard displayed on a screen or indirectly touching the keyboard with a stylus pen or the like, and use the input data. The electronic device includes a first transistor electrically connected to a reflective electrode and a photo sensor over a flexible substrate. A touch-input button is displayed as a still image on a first screen region of a display portion, and a video signal is output so that a moving image is displayed on a second screen region of the display portion. A video signal processing portion supplying different signals between the case where a still image is displayed on the display portion and the case where a moving image is displayed on the display portion is provided. After writing of a still image is performed, a display element control circuit is put in a non-operation state, whereby power consumption can be reduced.

TECHNICAL FIELD

The present invention relates to an electronic device having a circuit including a transistor, and also relates to an electronic system. For example, the present invention relates to an electronic device on which an electro-optical device typified by a liquid crystal display panel is mounted as a component.

BACKGROUND ART

In recent years, display devices such as electronic book readers have been actively developed. In particular, since a technique in which an image is displayed with the use of a display element having memory properties greatly contributes to reduction of power consumption, the technique has been actively developed (Patent Document 1).

In addition, a display device provided with a touch sensor has attracted attention. The display device provided with a touch sensor is called a touch panel, a touch screen, or the like (hereinafter also referred to simply as a touch panel). Further, a display device on which an optical touch sensor is mounted is disclosed in Patent Document 2.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.     2006-267982 -   [Patent Document 2] Japanese Published Patent Application No.     2001-292276

DISCLOSURE OF INVENTION

An object of an embodiment is to provide an electronic device which is lightweight and flexible and whose impact resistance is improved by using a flexible film as a base instead of a hard substrate such as a glass substrate.

Further, an object of an embodiment is to provide a driver circuit which is advantageous for energy saving of an electronic device whose power is limited, such as a portable information terminal.

One object of an embodiment is to provide a novel electronic device which is configured so that a user can read data regardless of a location, input data by directly touching a keyboard displayed on a screen or indirectly touching the keyboard with a stylus pen or the like, and use the input data. An object of an embodiment is to provide a pixel structure in which a light-receiving area of photo sensors and a pixel electrode area per unit area are increased in order to obtain an electronic device configured so that a user can read data and input data by touching a keyboard displayed on a screen.

In addition, an object of an embodiment is to provide a novel electronic device in which a still-image mode including display of a keyboard and a moving-image mode are realized on one screen.

In addition, one object of an embodiment is to reduce power consumption in such a manner that, in the case of the still-image mode, a still image is displayed on part of a display portion, then supply of power to display elements in a region where the still image is displayed is stopped, and a state in which the still image can be seen is kept for a long time after the stop of supply.

In an electronic device including a display portion in which an image is displayed using external light, the display portion has a touch-input function with the use of photo sensors, keyboard buttons are displayed on at least part of the display portion, and a user inputs data by touching a desired key, so that display corresponds to the desired key is performed on the display portion.

The photo sensors detect external light entering the display portion and a shadow which is made on part of the display portion (hereinafter, also referred to as a partial shadow of external light) when a user points a desired position on the display device. An input processing portion processes the position of a photo sensor which detects the partial shadow of external light on the display portion, as the coordinate position of touch input. A video signal processing portion outputs data corresponding to the coordinate of touch input, i.e., data of a keyboard, as image data to the display portion.

A first display region on which the keyboard is displayed displays a still image in a period in which a user inputs data with the keyboard displayed on the display portion. When the user inputs data, a second display region displays a moving image in a period in which letters or numerals corresponding to keys touched are written one after another or in a period in which conversion of letters is performed.

An embodiment of the present invention disclosed in this specification is an electronic device including a video signal processing portion which switches a first screen region of a display portion to a screen region on which touch input is performed or a screen region on which output is performed to display. Alternatively, the electronic device includes a video signal processing portion which supplies different signals to display elements of the display portion between the case where a still image is displayed on the display portion and the case where a moving image is displayed on the display portion. After writing of a still image is performed, a display element control circuit is put in a non-operation state, whereby power consumption can be reduced. In particular, a decoder circuit is preferably used as a scan line driver circuit.

A switching transistor included in a conventional active matrix display device has a problem in that off current is large and a signal written to a pixel leaks to disappear in the transistor even when the transistor is in an off state. According to an embodiment of the present invention, with the use of a transistor including an oxide semiconductor layer as a switching transistor, extremely small off current, specifically, off current density per channel width of 1 μm at room temperature can be less than or equal to 10 aA (1×10⁻¹⁷ A/μm), further, less than or equal to 1 aA (1×10⁻¹⁸ A/μm), or still further, less than or equal to 10 zA (1×10⁻²⁰ A/μm). In addition, in the pixel, holding time of an electric signal such as an image signal can be longer, and intervals of writing time can be set long. Therefore, with the use of the transistor including an oxide semiconductor, a period in which the display element control circuit in a non-operation state after writing of a still image is prolonged, whereby power consumption can be further reduced.

An embodiment of the present invention relating to a device to realize an electronic device is an electronic device comprising a display portion having a touch-input function; and a first transistor electrically connected to a reflective electrode, and a photo sensor over a flexible substrate. In the electronic device, the photo sensor comprises a photodiode, a second transistor including a gate signal line electrically connected to the photodiode, and a third transistor. In the electronic device, one of a source and a drain of the second transistor is electrically connected to a photo sensor reference signal line, the other of the source and the drain of the second transistor is electrically connected to one of a source and a drain of the third transistor, and the other of the source and the drain of the third transistor is electrically connected to a photo sensor output signal line.

With the above structure, at least one of the above problems can be resolved.

In the above structure, an oxide semiconductor layer of the second transistor overlaps with a reading signal line with a gate insulating layer provided therebetween, and the reading signal line overlaps with the reflective electrode that is a pixel electrode. With a pixel structure in which the reading signal line and the third transistor are provided below the reflective electrode, a light-receiving area of photo sensors and a pixel electrode area (hereinafter, referred to as a reflective electrode area) per unit area can be effectively used.

In addition, an embodiment of the invention is an electronic device comprising a display portion having a touch-input function; and a first transistor electrically connected to a first reflective electrode, a second transistor electrically connected to a second reflective electrode, and a photo sensor, over a flexible substrate. In the electronic device, the photo sensor comprises a photodiode, a third transistor including a gate signal line electrically connected to the photodiode, and a fourth transistor. In the electronic device, one of a source and a drain of the third transistor is electrically connected to a photo sensor reference signal line, the other of the source and the drain of the third transistor is electrically connected to one of a source and a drain of the fourth transistor, and the other of the source and the drain of the fourth transistor is electrically connected to a photo sensor output signal line. In the electronic device, an oxide semiconductor layer of the fourth transistor overlaps with the first reflective electrode, and the photo sensor reference signal line overlaps with the second reflective electrode.

The above structure is designed so that one light-receiving region of a photo sensor is provided between two reflective electrodes when the pixel structure is seen from above, whereby a light-receiving area of photo sensors and a reflective electrode area per unit area can be effectively used.

In the above structure, an oxide semiconductor layer of the third transistor overlaps with a reading signal line with a gate insulating layer provided therebetween, and the reading line overlaps with the first reflective electrode. With a pixel structure in which the reading signal line and the fourth transistor are provided below the first reflective electrode, a light-receiving area of photo sensors and a reflective electrode area per unit area can be effectively used.

In either of the above structures, one of the source and the drain of the fourth transistor overlaps with the first reflective electrode and the other of the source and the drain of the fourth transistor overlaps with the second reflective electrode. With such a pixel structure, a light-receiving area of photo sensors and a reflective area per unit area can be effectively used.

In addition, in the above structure, a color filter is provided to overlap with the first reflective electrode or the second reflective electrode, whereby full-color display can also be performed.

Further, a reflective liquid crystal device is advantageous for energy saving because displayed contents can be recognized with external light such as sunlight or illumination light even a backlight is not provided.

A portable information terminal in which a moving image and a still image are displayed on one screen can be realized. Driving and supply of a signal are performed in a different manner between in the case where a moving image is displayed on a screen region and the case where a still image is displayed on a screen region, and power consumption for displaying a moving image is reduced as compared to that for displaying a still image. In addition, since a reflective liquid crystal display device is employed, halftone display in grayscale with a wider range of gradation than in the case of an electrophoretic display device can be performed.

In addition, with the use of a potable information terminal whose weight is reduced by using a flexible substrate, a user can read data regardless of a location, and input data by touching a keyboard displayed on a screen, so that a result of the input can be displayed on the screen on which the keyboard is displayed.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B are external views illustrating an embodiment of the present invention;

FIG. 2 is a block diagram illustrating an embodiment of the present invention;

FIG. 3 is an equivalent circuit diagram of a pixel, illustrating an embodiment of the present invention;

FIG. 4 is a schematic view of a driver circuit for photo sensors, illustrating an embodiment of the present invention;

FIG. 5 is a timing chart showing an embodiment of the present invention;

FIG. 6 is an example of a plan view of a pixel, illustrating an embodiment of the present invention;

FIG. 7 is an example of a plan view showing a positional relation between a reflective electrode and a black matrix, illustrating an embodiment of the present invention;

FIGS. 8A and 8B are an example of cross-sectional views illustrating an embodiment of the present invention;

FIG. 9 is a schematic view of a liquid crystal display module, illustrating an embodiment of the present invention;

FIGS. 10A and 10B are an external view and a block diagram of a display device which is an embodiment of the present invention;

FIGS. 11A and 11B are a photograph of a cross section of part of a photodiode and a schematic view thereof;

FIG. 12 is a photograph showing a state of a display panel displaying an image;

FIGS. 13A to 13C are an example of a cross-sectional views illustrating an embodiment of the present invention;

FIGS. 14A and 14B are schematic views of an electronic book reader, illustrating an embodiment of the present invention;

FIG. 15 is a block diagram of a driver circuit, illustrating an embodiment of the present invention;

FIG. 16 shows a relation among data lines with respect to time;

FIG. 17 is a diagram of a circuit forming the inside of a block; and

FIG. 18 shows time and respective potentials of nodes.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, the present invention is not limited to the description below, and it is easily understood by those skilled in the art that modes and details disclosed herein can be modified in various ways without departing from the spirit and the scope of the present invention. Therefore, the present invention is not construed as being limited to description of the embodiments.

Embodiment 1

In this embodiment, an example of an electronic device including a display portion in which an image is displayed using external light is described with reference to FIGS. 1A and 1B.

A display portion 1032 of an electronic device 1030 has a touch-input function with the use of photo sensors, in which a plurality of keyboard buttons 1031 is displayed on a region 1033 in the display portion as illustrated in FIG. 1A. The display portion 1032 indicates the entire display region and includes the region 1033 in the display portion. A user inputs data by touching desired keyboard buttons, whereby a result of the input is displayed on the display portion 1032.

Since the region 1033 in the display portion displays a still image, power consumption can be reduced by putting a display element control circuit in a non-operation state in a period other than writing time.

An example of the usage of the electronic device 1030 is described. For example, letters are input by successively touching the keyboard buttons displayed on the region 1033 in the display portion with user's fingers or without contact, and text which is displayed as a result of the input is displayed on a region other than the region 1033 in the display portion. After a set period of time in which an output signal of the photo sensor is not detected since the user removes his/her fingers from the keyboard of the screen, the keyboard displayed on the region 1033 in the display portion is removed automatically and the input text is displayed also on the region 1033 in the display portion, so that the user can see the input text displayed on all the region of the screen. In the case where input is performed again, the keyboard buttons can be displayed on the region 1033 in the display portion again by touch of the display portion 1032 with the user's finger or detection of an output signal of a photo sensor without contact, and input of letters can be performed.

Alternatively, the keyboard can be removed not automatically but by pushing a switch 1034 by the user so that a still image can be displayed on the entire display portion 1032 as illustrated in FIG. 1B. In addition, even when the power is turned off by pushing a power supply switch 1035, the still image can be held for a long time. Further, the keyboard can be displayed by pushing a keyboard display switch 1036 to be in a state where touch-input can be performed.

In addition, the switch 1034, the power supply switch 1035, and the keyboard display switch 1036 may each be displayed on the display portion 1032 as a switch button. Each operation may be performed by touching the displayed switch button.

Further, without limitation to the structure in which the region 1033 in the display portion displays a still image, the region 1033 in the display portion may display a moving image temporarily or partly. For example, a position where the keyboard buttons are displayed may be changed temporarily in accordance with user's taste, or when input is performed without contact, only display of a keyboard button corresponding to a letter which is input may be partly changed in order to confirm that whether the input is performed.

The electronic device 1030 includes at least a battery, and preferably includes a memory for storing data (e.g., a flash memory circuit, an SRAM circuit, or a DRAM circuit), a central processing unit (CPU), or a logic circuit. With a CPU or a memory, various kinds of software can be installed and thus, the electronic device 1030 can have part or all of the functions of a personal computer.

In addition, by providing a gradient detection portion such as a gyroscope or a triaxial acceleration sensor in the electronic device 1030, a function used in the electronic device 1030, in particular, a function relating to display and input on the display portion can be switched by an arithmetic circuit in accordance with a signal from the gradient detection portion. Therefore, unlike an electronic device with an input key which has predetermined kind, size, or arrangement, such as a built-in operation key, the electronic device 1030 can improve convenience for users.

Next, an example of a display panel included in the display portion 1032 is described with reference to FIG. 2. A display panel 100 includes a pixel circuit 101, a display element control circuit, and a photo sensor control circuit. The pixel circuit 101 includes a plurality of pixels 103 and 104 and a plurality of photo sensors 106 which are arranged in a matrix of rows and columns. Each of the pixels 104 and 103 includes one display element. Although in this embodiment, one photo sensor 106 is provided between the pixel 103 and the pixel 104 and the number of the photo sensors is half of the number of the pixels, an embodiment is not limited thereto. One photo sensor may be provided per one pixel so that the number of the photo sensors is the same as the number of the pixels. Alternatively, the number of the photo sensors may be one third of the number of the pixels.

A display element 105 includes a liquid crystal element including a transistor, a storage capacitor, and a liquid crystal layer, or the like. The transistor has a function of controlling injection or discharge of charge to/from the storage capacitor. The storage capacitor has a function of retaining charge which corresponds to voltage applied to the liquid crystal layer. Taking advantage of the change in the direction of a polarization due to a voltage application to the liquid crystal layer, contrast (gray scale) of light passing through the liquid crystal layer is made, so that image display is realized. External light (sunlight or illumination light) which enters from a surface side of a liquid crystal display device is used as the light passing through the liquid crystal layer. There is no particular limitation on the liquid crystal layer, and a known liquid crystal material (typically, a nematic liquid crystal material or a cholesteric liquid crystal material) may be used. For example, polymer dispersed liquid crystal (PDLC) or polymer network liquid crystal (PNLC) may be used for the liquid crystal layer so that white display (light display) is performed using scattered light by liquid crystal. When PDLC or PNLC is used for the liquid crystal layer, a polarizing plate is not needed and display close to paper is possible, which is eye-friendly and causes less feeling tired.

Further, the display element control circuit is a circuit configured to control the display elements 105 and includes a display element driver circuit 107 which inputs a signal to the display elements 105 through signal lines (also referred to as source signal lines) such as video data signal lines, and a display element driver circuit 108 which inputs a signal to the display elements 105 through scan lines (also referred to as gate signal lines).

For example, the display element driver circuit 108 on the scan line side has a function of selecting the display elements included in the pixels placed in a particular row. The display element driver circuit 107 on the driving the signal line side has a function of applying a predetermined potential to the display elements included in the pixels placed in a selected row. Note that in the display element to which the display element driver circuit 108 on the scan line side applies high potential, the transistor is in a conduction state, so that the display element is supplied with charge from the display element driver circuit 107 on the signal line side

The photo sensor 106 includes a transistor and a light-receiving element which has a function of generating an electrical signal when receiving light, such as a photodiode.

The photo sensor control circuit is a circuit configured to control the photo sensors 106 and includes a photo sensor reading circuit 109 on the signal line side for a photo sensor output signal line, a photo sensor reference signal line, or the like, and a photo sensor driver circuit 110 on the scan line side. The photo sensor driver circuit 110 on the scan line side has a function of performing reset operation and selecting operation on the photo sensors 106 included in the pixels placed in a particular row, which is described below. Further, the photo sensor reading circuit 109 on the signal line side has a function of taking out an output signal of the photo sensors 106 included in the pixels in the selected row.

A circuit diagram of the pixel 103, the photo sensor 106, and the pixel 104 is described in this embodiment with reference to FIG. 3. The pixel 103 includes the display element 105 including a transistor 201, a storage capacitor 202, and a liquid crystal element 203. The photo sensor 106 includes a photodiode 204, a transistor 205, and a transistor 206. The pixel 104 includes a display element 125 including a transistor 221, a storage capacitor 222, and a liquid crystal element 223.

A gate of the transistor 201 is electrically connected to a gate signal line 207, one of a source and a drain of the transistor 201 is electrically connected to a video data signal line 210, and the other of the source and the drain of the transistor 201 is electrically connected to one electrode of the storage capacitor 202 and one of electrodes of the liquid crystal element 203. The other electrode of the storage capacitor 202 is electrically connected to a capacitor wiring 214 and held at a fixed potential. The other electrode of the liquid crystal element 203 is held at a fixed potential. The liquid crystal element 203 is an element including a pair of electrodes and a liquid crystal layer provided between the pair of electrodes.

When “H” (high-level potential) is applied to the gate signal line 207, the transistor 201 applies the potential of the video data signal line 210 to the storage capacitor 202 and the liquid crystal element 203. The storage capacitor 202 holds the applied potential. The liquid crystal element 203 changes light transmittance in accordance with the applied potential.

One of electrodes of the photodiode 204 is electrically connected to a photodiode reset signal line 208, and the other electrode of the photodiode 204 is electrically connected to a gate of the transistor 205. One of a source and a drain of the transistor 205 is electrically connected to a photo sensor reference signal line 212, and the other of the source and the drain of the transistor 205 is electrically connected to one of a source and a drain of the transistor 206. A gate of the transistor 206 is electrically connected to a reading signal line 209, and the other of the source and the drain of the transistor 206 is electrically connected to a photo sensor output signal line 211.

A gate of the transistor 221 is electrically connected to a gate signal line 227, one of a source and a drain of the transistor 221 is electrically connected to the video data signal line 210, and the other of the source and the drain of the transistor 221 is electrically connected to one of electrodes of the storage capacitor 222 and one of electrodes of the liquid crystal element 223. The other electrode of the storage capacitor 222 is electrically connected to a capacitor wiring 224 and held at a fixed potential. The other electrode of the liquid crystal element 223 is held at a fixed potential. The liquid crystal element 223 includes a pair of electrodes and a liquid crystal layer provided between the pair of electrodes.

Next, an example of a structure of the photo sensor reading circuit 109 is described with reference to FIG. 3 and FIG. 4. For example, the display portion includes pixels provided in 1024 rows and 768 columns. One display element is provided in one pixel in each row and each column and one photo sensor is provided between two pixels in two rows and one column. That is, the display elements are provided in 1024 rows and 768 columns, and the photo sensors are provided in 512 rows and 768 columns. In addition, this embodiment describes an example in which output to the outside of the display device is performed in the case where photo sensor output signal lines in two columns are regarded as one pair. That is, one output is obtained from two photo sensors provided between four pixels in two rows and two columns.

FIG. 3 illustrates a circuit configuration of pixels, in which two pixels and one photo sensor for two rows and one column are illustrated. One display element is provided in one pixel and one photo sensor is provided between two pixels. FIG. 4 illustrates a circuit configuration of the photo sensor reading circuit 109, in which some photo sensors are illustrated for explanation.

As illustrated in FIG. 4, an example of a driving method is considered, in which a scan line driver circuit for photo sensors drives pixels for four rows (that is, photo sensors for two rows) concurrently, and shifts selected rows by one row including photo sensors corresponding to pixels for two rows. Here, photo sensors in each row are continually selected in a period in which the scan line driver circuit shifts selected rows twice. Such a driving method facilitates improving frame frequency at the time of imaging by a photo sensor. In particular, it is advantageous in the case of a large-sized display device. Note that outputs of photo sensors in two rows are superimposed on the photo sensor output signal line 211 at one time. All of the photo sensors can be driven by repeating shift of selected rows 512 times.

As illustrated in FIG. 4, in the photo sensor reading circuit 109, one selector is provided per pixels for 24 rows. The selector selects 1 pair from 12 pairs of photo sensor output signal lines 211 (1 pair corresponds to photo sensor output signal lines 211 for two columns) in the display portion and obtains an output. In other words, the photo sensor reading circuit 109 includes 32 selectors in total and obtains 32 outputs at one time. Selection is performed on all of the 12 pairs in each selector, whereby 384 outputs which correspond to one row of photo sensors can be obtained in total. The selector selects 1 pair from the 12 pairs every time selected rows are shifted by the scan line driver circuit of photo sensors, whereby outputs from all of the photo sensors can be obtained.

In this embodiment, as illustrated in FIG. 4, the following structure is considered: the photo sensor reading circuit 109 on the signal line side takes out outputs of photo sensors, which are analog signals, to the outside of the display device, and the outputs are amplified with the use of an amplifier provided outside the display device and converted to digital signals with the use of an AD converter. Needless to say, the following structure may be employed: the AD converter is mounted on a substrate over which the display device is provided, and the outputs of photo sensors are converted to digital signals and then the digital signals are taken out to the outside of the display device.

In addition, operation of individual photo sensors is realized by repeating reset operation, accumulating operation, and selecting operation. The “reset operation” refers to operation in which the potential of the photodiode reset signal line 208 is set to “H”. When the reset operation is performed, the photodiode 204 is brought into conduction, and the potential of a gate signal line 213 to which the gate of the transistor 205 is connected is set to “H”.

The “accumulating operation” refers to operation in which the potential of the photodiode reset signal line 208 is set to “L” (low-level potential) after the reset operation. Further, the “selecting operation” refers to operation in which the potential of the reading signal line 209 is set to “H” after the accumulating operation.

When the accumulating operation is performed, the potential of the gate signal line 213 to which the gate of the transistor 205 is connected is reduced as light with which the photodiode 204 is irradiated is stronger, and a channel resistance of the transistor 205 is increased. Therefore, when the selecting operation is performed, a current which flows to the photo sensor output signal line 211 through the transistor 206 is small. On the other hand, as the light with which the photodiode 204 is irradiated is weaker at the time of the accumulating operation, a current which flows to the photo sensor output signal line 211 through the transistor 206 is increased at the time of the selecting operation.

In this embodiment, when the reset operation, the accumulating operation, and the selecting operation are performed on the photo sensors, a partial shadow of external light can be detected. In addition, when image processing or the like is performed on the detected shadow appropriately, a position where a finger, a stylus pen, or the like touches the display device can be recognized. Operation corresponding to the position where the display device is touched, for example, as for input of letters, kinds of letters are regulated in advance, so that desired letters can be input.

Note that in the display device in this embodiment, the partial shadow of external light is detected by the photo sensors. Therefore, even if a finger, a stylus pen, or the like does not touch the display device physically, when the finger, the stylus pen, or the like gets close to the display device without contact and a shadow is formed, detection of the shadow is possible. Hereinafter, “a finger, a stylus pen, or the like touches the display device” includes the case where the finger, the stylus pen, or the like is close to the display device without contact.

With the above structure, the display portion 1032 can have a touch-input function.

In the case where touch input is performed, the display device has a structure in which an image partly including a still image such as a keyboard is displayed and input is performed by touching a position where a keyboard or a desired letter is displayed with a finger or a stylus pen, whereby operability is improved. In the case where such a display device is realized, power consumption in the display device can be considerably reduced in the following manner. That is, in a first screen region where the still image is displayed on the display portion, it is effective that supply of power to display elements in the first screen region is stopped after the still image is displayed and that a state in which the still image can be seen is kept for a long time after the stop of supply. In a second screen region that is the rest of the display portion, a result from the touch input is displayed, for example. The display element control circuit is in a non-operation state in a period other than the time of updating the displayed image in the second screen region, whereby power can be saved. A driving method which enables the above control is described below.

For example, FIG. 5 shows a timing chart of a shift register of the scan line driver circuit in the display device including the display portion in which the display elements are arranged in 1024 rows and 768 columns. A period 61 in FIG. 5 corresponds to one cycle period (64.8 μsec) of a clock signal. A period 62 corresponds to a period (8.36 msec) which is needed for finishing writing of the display elements from a 1st to 512th rows corresponding to the second screen region. A period 63 corresponds to one frame period (16.7 msec).

Here, the shift register of the scan line driver circuit is a four-phase-clock-type shift register which is operated by a first clock signal CK1 to a fourth clock signal CK4. In addition, the first clock signal CK1 to the fourth clock signal CK4 are sequentially delayed by a quarter of one cycle period. When a start pulse signal GSP is set to “H”, a gate signal line G1 in a 1st row to a gate signal line G512 in a 512th row are set to “H” in sequence with a delay of a quarter of one cycle period. In addition, each of the gate signal lines is set at “H” during half of one cycle period, and two gate signal lines in successive rows are set at “H” concurrently during quarter of one cycle period.

Here, the display elements in each row are continuously selected in a period in which the scan line driver circuit shifts selected rows twice. When data of the displayed image are input in the latter half of a period in which the display elements in the row are selected, the displayed image can be updated.

Here, in a period other than a period in which the displayed image by the display elements from the 1st row to the 512th row which correspond to the second screen region is updated, the display element control circuit is in a non-operation state. That is, the displayed image by display elements in a 513th row to a 1024th row corresponding to the first screen region is not updated and the display element control circuit is in a non-operation state.

The non-operation state of the display element control circuit can be realized by stopping the clock signal (keeping a clock signal at a potential “L”) as shown in FIG. 5. It is effective to stop of supply of power supply voltage at the same time as the stop of the clock signal.

In addition, supply of a clock signal and a start pulse signal may be stopped also in the driver circuit on the source in a period in which the display elements corresponding to the second screen region are not selected, that is, in a period in which the displayed image is not updated. In such a manner, power can be further saved.

Further, power can be saved by using a decoder instead of the shift register of the scan line driver circuit.

Embodiment 2

A pixel structure corresponding to FIG. 2 and FIG. 3 described in Embodiment 1 is described in this embodiment with reference to FIG. 6, FIG. 7, and FIGS. 8A and 8B. Note that the portions which are the same as those in FIG. 2 and FIG. 3 are described using the same reference numerals in the description for FIG. 6, FIG. 7, and FIGS. 8A and 8B.

FIG. 6 illustrates an example of a plan view of a pixel, corresponding to the circuit diagram of FIG. 3. In addition, FIG. 8A illustrates a state at the time before an electrode of a photodiode is formed. Note that a cross-sectional view taken along a chain line A-B and a cross-sectional view taken along a chain line C-D in FIG. 6 correspond to FIG. 8A.

First, a conductive film is formed over a substrate 230. Then, gate signal lines 207, 213, and 227, a capacitor wiring 224, a photodiode reset signal line 208, a reading signal line 209, and a photo sensor reference signal line 212 are formed through a first photolithography step with the use of a first light-exposure mask. In this embodiment, a glass substrate is used as the substrate 230.

An insulating film serving as a base film may be provided between the substrate 230 and the conductive film. The base film has a function of preventing diffusion of an impurity element from the substrate 230. The base film can be formed with a single-layer structure or a layered structure including one or more of a silicon nitride film, a silicon oxide film, a silicon nitride oxide film, and a silicon oxynitride film.

The conductive film can be formed with a single-layer structure or a layered structure including a metal material such as molybdenum, titanium, tantalum, tungsten, aluminum, copper, neodymium, or scandium, or an alloy material which contains any of these metal materials as its main component.

Then, an insulating layer covering these wirings is formed, and selective etching is performed through a second photolithography step with the use of a second light-exposure mask so that an insulating layer 231 remains only in a portion intersecting a wiring which is to be formed later. In this embodiment, a silicon oxynitride film with a thickness of 600 nm is used as the insulating layer 231.

Next, a gate insulating layer 232 and an oxide semiconductor film are formed, and then, a first oxide semiconductor layer 233, a second oxide semiconductor layer 253, a third oxide semiconductor layer 255, and a fourth oxide semiconductor layer 256 are formed through a third photolithography step with the use of a third light-exposure mask. The first oxide semiconductor layer 233, the second oxide semiconductor layer 253, the third oxide semiconductor layer 255, and the fourth oxide semiconductor layer 256 overlap with the gate signal line 227, the gate signal line 207, the reading signal line 209, and the gate signal line 213, respectively with the gate insulating layer 232 provided therebetween. In this embodiment, a silicon oxynitride film with a thickness of 100 nm is used as the gate insulating layer 232, and an In—Ga—Zn—O film with a thickness of 30 nm is used as the oxide semiconductor film.

An oxide thin film represented by a chemical formula InMO₃(ZnO)_(m) (m>0) can be used for the first oxide semiconductor layer 233, the second oxide semiconductor layer 253, the third oxide semiconductor layer 255, and the fourth oxide semiconductor layer 256. Here, M represents one or more metal elements selected from Ga, Al, Mn, and Co. For example, M can be Ga, Ga and Al, Ga and Mn, Ga and Co, or the like. Further, SiO₂ may be contained in the above oxide thin film.

As the target for forming the oxide thin film by a sputtering method, for example, an oxide target having a composition ratio of In₂O₃:Ga₂O₃:ZnO=1:1:1 [molar ratio] is used to form an In—Ga—Zn—O film. Without limitation on the material and the component of the target, for example, an oxide target having a composition ratio of In₂O₃:Ga₂O₃:ZnO=1:1:2 [molar ratio] may be used. Note that in this specification, for example, an In—Ga—Zn—O film means an oxide film including indium (In), gallium (Ga), and zinc (Zn), and there is no particular limitation on the stoichiometric proportion.

Next, the oxide semiconductor layers are subjected to first heat treatment. The oxide semiconductor layers can be dehydrated or dehydrogenated by this first heat treatment. The temperature of the first heat treatment is higher than or equal to 400° C. and lower than or equal to 750° C., preferably higher than or equal to 400° C. and lower than the strain point of the substrate. In embodiment, a rapid thermal anneal (RTA) apparatus is used, heat treatment is performed in a nitrogen atmosphere at 650° C. for six minutes, the substrate is introduced, without exposure to the air, into an electric furnace that is one kind of a heat treatment apparatus, and heat treatment is performed in a nitrogen atmosphere at 450° C. for one hour for the oxide semiconductor layers. Then, the substrate is transferred into the deposition chamber of the oxide semiconductor layers so as not to be exposed to the air in order to prevent mixing of water or hydrogen to the oxide semiconductor layers, whereby the oxide semiconductor layers are obtained.

Next, the gate insulating layer 232 is selectively removed through a fourth photolithography step with the use of a fourth light-exposure mask, so that an opening reaching the gate signal line 213 and an opening reaching the photodiode reset signal line 208 are formed.

Next, a conductive film is formed over the gate insulating layer 232 and the oxide semiconductor layer. The conductive film can be formed using a metal film containing an element selected from Al, Cr, Cu, Ta, Ti, Mo, and W as its component, an alloy film containing a nitride of any of these elements as its component, an alloy film containing a combination of any of these elements, or the like. In this embodiment, the conductive film has a three-layer structure in which a Ti film with a thickness of 100 nm, an Al film with a thickness of 400 nm, and a Ti film with a thickness of 100 nm are stacked. Then, a resist mask is formed over the conductive film through a fifth photolithography step using a fifth light-exposure mask and etching is selectively performed, thereby forming a video data signal line 210, a photo sensor output signal line 211, and electrode layers 234, 235, 254, 257, 258, and 259.

Note that a transistor 221 illustrated in FIG. 3 includes the first oxide semiconductor layer 233 and the electrode layer 234 serving as a source electrode layer or a drain electrode layer as illustrated in FIG. 6. In addition, as illustrated in FIG. 6 and FIG. 8A, the electrode layer 234, the gate insulating layer 232 serving as a dielectric, and the capacitor wiring 224 form a storage capacitor 222. Further, as illustrated in FIG. 6, a transistor 201 includes the second oxide semiconductor layer 253 and the electrode layer 254 serving as a source electrode layer or a drain electrode layer.

Furthermore, a transistor 206 that is one of the elements included in a photo sensor 106 in FIG. 3 includes the third oxide semiconductor layer 255 and the electrode layer 257 serving as a source electrode layer or a drain electrode layer as illustrated in FIG. 6. In addition, a transistor 205 includes the fourth oxide semiconductor layer 256 and the electrode layer 257 or the electrode layer 258 which serves as a source electrode layer or a drain electrode layer as illustrated in FIG. 6. As illustrated in FIG. 8A, the gate signal line 213 of the transistor 205 is electrically connected to the electrode layer 236 through the opening of the gate insulating layer 232.

Next, second heat treatment is performed in an inert gas atmosphere or oxygen gas atmosphere (preferably at a temperature higher than or equal to 200° C. and lower than or equal to 400° C., for example, higher than or equal to 250° C. and lower than or equal to 350° C.). In this embodiment, the second heat treatment is performed at 300° C. for one hour under a nitrogen atmosphere. Through the second heat treatment, part of the oxide semiconductor layer (a channel formation region) is heated while being in contact with the insulating layer.

Next, an insulating layer 237 serving as a protective insulating layer is formed, and a sixth photolithography step is performed with the use of a sixth light-exposure mask, so that an opening reaching the electrode layer 235, an opening reaching the electrode layer 234, and an opening reaching the electrode layer 236 are formed. In this embodiment, a silicon oxide film with a thickness of 300 nm obtained by a sputtering method is used as the insulating layer 237.

Next, a p-layer 238, an i-layer 239, and an n-layer 240 are stacked by a plasma CVD method. In this embodiment, a microcrystalline silicon film containing boron with a thickness of 60 nm is used as the p-layer 238, an amorphous silicon film with a thickness of 400 nm is used as the i-layer 239, and a microcrystalline silicon film containing phosphorus with a thickness of 80 nm is used as the n-layer 240. The p-layer 238, the i-layer 239, and the n-layer 240 are selectively etched through a seventh photolithography step with the use of a seventh light-exposure mask, and further, as illustrated in FIG. 8A, the periphery of the n-layer 240 and part of the i-layer 239 are selectively removed. FIG. 8A is a cross-sectional view up to this stage and a plan view thereof corresponds to FIG. 6.

Next, an eight photolithography step is performed in which a photosensitive organic resin layer is formed, a region in which an opening is to be formed is exposed to light with the use of an eighth light-exposure mask, a region to have an uneven shape is exposed to light with the use of a ninth light-exposure mask, development is performed, and an insulating layer 241 partly having an uneven shape is formed. In this embodiment, an acrylic resin with a thickness of 1.5 μm is used as the photosensitive organic resin layer.

Then, a conductive film having reflectivity is deposited, and a ninth photolithography step is performed with the use of a tenth light-exposure mask, so that a reflective electrode layer 242 and a connection electrode layer 243 are formed. Note that the reflective electrode layer 242 and the connection electrode layer 243 are illustrated in FIG. 8B. Al, Ag, or an alloy thereof such as aluminum containing Nd or an Ag—Pd—Cu alloy is used as the conductive film having reflectivity. In this embodiment, a stack of a Ti film with a thickness of 100 nm and an Al film with a thickness of 300 nm provided thereover is used as the conductive film having reflectivity. After the ninth photolithography step, third heat treatment is performed. In this embodiment, the third heat treatment is performed at 250° C. for one hour in a nitrogen atmosphere.

Through the above steps, a transistor electrically connected to the reflective electrode layer 242 and a photodiode electrically connected to the gate signal line 213 through the connection electrode layer 243 can be formed over one substrate through the nine photolithography steps with the use of the ten light-exposure masks in total.

Note that a cross-sectional photograph of the periphery portion of the photodiode is shown in FIG. 11A. FIG. 11A shows a cross section of a photodiode 204 and FIG. 11B illustrates a schematic view of the cross section.

An alignment film 244 covering the reflective electrode layer 242 is formed. A cross-sectional view at this stage corresponds to FIG. 8B. Note that the same reference numeral is used for common parts in FIG. 8B and FIG. 11B. Thus, an active matrix substrate can be manufactured.

A counter substrate to be bonded to the active matrix substrate is prepared. Over the counter substrate, a light-blocking layer (also referred to as a black matrix) and a light-transmitting conductive film are formed and a columnar spacer using an organic resin is formed. Then, an alignment film is formed to cover them.

The counter substrate is attached to the active matrix substrate with a sealant, and a liquid crystal layer is sandwiched between the pair of substrates. The light-blocking layer of the counter substrate is provided so as not to overlap with a display region of the reflective electrode layer 242 and a sensing region of the photodiode. The columnar spacer provided on the counter substrate is positioned so as to overlap with the electrode layers 251 and 252. Since the columnar spacer overlaps with the electrode layers 251 and 252, the pair of substrates is held at a certain distance. The electrode layers 251 and 252 can be formed in the same step as that of the electrode layer 234; thus, it is not necessary to increase the number of masks. The electrode layers 251 and 252 are not electrically connected to anywhere and are in a floating state.

A plan view of the pixels for the pair of substrate which are attached to each other in this manner corresponds to FIG. 7. In FIG. 7, a region which does not overlap with the black matrix serves as a light-receiving region of a photo sensor and a reflective electrode region. The proportion of the area of the reflective electrode in a unit area (120 μm×240 μm) illustrated in FIG. 7 is 77.8%. The area of the light-receiving region of the photo sensor is approximately 1140 μm². In addition, since the reflective electrode layer 242 is provided over the photosensitive organic resin layer having an uneven portion, the reflective electrode layer 242 has a random plane pattern as illustrated in FIG. 7. The surface shape of the photosensitive organic resin layer is reflected on a surface of the reflective electrode layer 242 so that the surface of the reflective electrode layer 242 has an uneven shape; thus, specular reflection is prevented. Note that in FIG. 7, a recessed portion 245 of the reflective electrode layer 242 is also illustrated. The periphery of the recessed portion 245 is positioned inside the periphery of the reflective electrode layer, and the photosensitive organic resin layer below the recessed portion 245 has a smaller thickness than other regions.

If necessary, a surface of the counter substrate where external light enters may be provided with an optical film such as a retardation film for adjusting phase difference, a film having a polarization function, an anti-reflection plate, or a color filter.

FIG. 12 shows a photograph of a panel which was actually manufactured, in which a video signal is input to the panel through an FPC to perform display. A half screen of the panel displayed a still image and the other half of the screen displayed a moving image. When the half of the screen of the panel displays a still image and the other half of the screen displays a moving image, power consumption can be reduced. In addition, keyboard buttons are displayed as illustrated in FIG. 1A by touching the screen with a finger and data is input by touching portions where keyboard buttons are displayed with fingers, whereby letters corresponding to the keyboard buttons can be displayed on the display region. Further, when photo sensors are used and a shadow of a finger can be sensed with a sufficient amount of external light even in a non-contact state in which the finger is close to the screen but not touches the screen, screen operation without contact can be performed.

Embodiment 3

In this embodiment, an example of a liquid crystal display module capable of full-color display in which a color filter is provided will be described.

FIG. 9 illustrates a structure of a liquid crystal display module 190. The liquid crystal display module 190 includes a display panel 120 in which liquid crystal elements are provided in a matrix, and a polarizing plate and a color filter 115 which overlap with the display panel 120. In addition, flexible printed circuits (FPCs) 116 a and 116 b serving as external input terminals are electrically connected to a terminal portion provided in the display panel 120. The display panel 120 has the same structure as the display panel 100 described in Embodiment 1. Note that since full-color display is employed, the display panel 120 uses three display elements of a red display element, a green display element, and a blue display element and has a circuit configuration in which the three display elements are supplied with respective video signals different from each other.

Further, FIG. 9 schematically illustrates a state in which external light 139 is transmitted through the liquid crystal elements over the display panel 120 and reflected at the reflective electrode. For example, in a pixel overlapping with a red region of the color filter, the external light 139 is transmitted through the color filter 115 and then through the liquid crystal layer, reflected at the reflective electrode, and transmitted again through the color filter 115 to be extracted as red light. FIG. 9 schematically illustrates light 135 of three colors by arrows (R, G, and B). The intensity of the light which is transmitted through the liquid crystal elements is modulated by an image signal. Therefore, a viewer can capture an image by reflection light of the external light 139.

In addition, the display panel 120 includes photo sensors and has a touch-input function. When the color filter also overlaps with a light-receiving region of photo sensors, the display panel 120 can have a function of a visible light sensor. Further, in order to improve the optical sensitivity of the photo sensor, a large amount of incident light is taken in. Therefore, an opening may be provided in the color filter in a region overlapping with the light-receiving region of photo sensors so that the light-receiving region of photo sensors and the color filter do not overlap with each other.

This embodiment can be freely combined with Embodiment 1 or Embodiment 2.

Embodiment 4

In this embodiment, an example of an electronic device including the liquid crystal display device described in any of the above embodiments is described.

FIG. 10A illustrates an electronic book reader (also referred to as an e-book reader) which can include housings 9630, a display portion 9631, operation keys 9632, a solar cell 9633, and a charge and discharge control circuit 9634. The electronic book reader is provided with the solar cell 9633 and a display panel so that the solar cell 9633 and the display panel can be opened and closed freely. In the electronic book reader, power from the solar cell is supplied to the display panel or a video signal processing portion. The electronic book reader illustrated in FIG. 10A can have a function of displaying various kinds of data (e.g., a still image, a moving image, and a text image), a function of displaying a calendar, a date, the time, or the like on the display portion, a touch-input function of operating or editing the information displayed on the display portion by touch input, a function of controlling processing by various kinds of software (programs), and the like. Note that in FIG. 10A, a structure including a battery 9635 and a DCDC converter (hereinafter abbreviated as a converter 9636) is illustrated as an example of the charge and discharge control circuit 9634.

The display portion 9631 is a reflective liquid crystal display device having a touch-input function with the use of photo sensors and is used in a comparatively bright environment. Therefore, the structure illustrated in FIG. 10A is preferable because power generation by the solar cell 9633 and charge in the battery 9635 can be performed effectively. Note that a structure in which the solar cell 9633 is provided on each of a surface and a rear surface of the housing 9630 is preferable in order to charge the battery 9635 efficiently. When a lithium ion battery is used as the battery 9635, there is an advantage of downsizing or the like.

The structure and the operation of the charge and discharge control circuit 9634 illustrated in FIG. 10A are described with reference to a block diagram in FIG. 10B. The solar cell 9633, the battery 9635, the converter 9636, the converter 9637, switches SW1 to SW3, and the display portion 9631 are shown in FIG. 10B, and the battery 9635, the converter 9636, the converter 9637, and the switches SW1 to SW3 correspond to the charge and discharge control circuit 9634.

First, an example of operation in the case where power is generated by the solar cell 9633 using external light is described. The voltage of power generated by the solar cell is raised or lowered by the converter 9636 so that the power has a voltage for charging the battery 9635. Then, when the power from the solar cell 9633 is used for the operation of the display portion 9631, the switch SW1 is turned on and the voltage of the power is raised or lowered by the converter 9637 so as to be a voltage needed for the display portion 9631. In addition, when display on the display portion 9631 is not performed, the switch SW1 is turned off and a switch SW2 is turned on so that charge of the battery 9635 may be performed.

Note that although the solar cell 9633 is described as an example of a means for charge, charge of the battery 9635 may be performed with another means. In addition, a combination of the solar cell 9633 and another means for charge may be used.

This embodiment can be implemented in appropriate combination with any of the structures described in the other embodiments.

Embodiment 5

In this embodiment, an example in which a transistor and a photo sensor are formed over a glass substrate, and transferred to and mounted on a flexible substrate. Note that here, cross-sectional views of steps for forming the transistor are illustrated in FIGS. 13A to 13C. In FIGS. 13A to 13C, the same steps as those of Embodiment 2 and detail description of the structure of a photodiode or the like are omitted and the same portions as those of FIGS. 8A and 8B are denoted by the same reference numerals.

First, a separation layer 260 is deposited over a substrate 230 by a sputtering method, and an oxide insulating film 261 functioning as a base film is formed thereover. Note that as the substrate 230, a glass substrate, a quartz substrate, or the like is used. The oxide insulating film 261 is formed using a material such as silicon oxide, silicon oxynitride (SiO_(x)N_(y)) (x>y>0), or silicon nitride oxide (SiN_(x)O_(y))(x>y>0) by a PCVD method, a sputtering method, or the like.

The separation layer 260 may be formed using a metal film, a layered structure of a metal film and a metal oxide film, or the like. As a metal film, either a single layer or stacked layers of a film formed using an element selected from tungsten (W), molybdenum (Mo), titanium (Ti), tantalum (Ta), niobium (Nb), nickel (Ni), cobalt (Co), zirconium (Zr), zinc (Zn), ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), and iridium (Ir), or a film formed using an alloy material or a compound material containing the element as its main component. For example, when a tungsten film is provided as a metal film by a sputtering method, a CVD method, or the like, a metal oxide film formed of tungsten oxide can be formed on a surface of the tungsten film by application of plasma treatment to the tungsten film. In addition, for example, after a metal film (e.g., tungsten) is formed, an insulating film formed of silicon oxide or the like may be formed over the metal film by a sputtering method, and also metal oxide (e.g., tungsten oxide over tungsten) may be formed over the metal film. Further, high-density plasma treatment with the use of a high-density plasma apparatus may be performed as the plasma treatment. Furthermore, besides a metal oxide film, a metal nitride film or a metal oxynitride film may be used. In this case, plasma treatment or heat treatment is performed on the metal film in a nitrogen atmosphere or an atmosphere of nitrogen and oxygen.

Next, a conductive film is formed over the oxide insulating film 261. Then, in a similar manner to Embodiment 2, a gate signal line 227, a capacitor wiring 224, a photodiode reset signal line, a reading signal line, and a photo sensor reference signal line are formed through a first photolithography step with the use of a first light-exposure mask.

Subsequent steps are performed in accordance with Embodiment 2, whereby the transistor and a reflective electrode layer 242 are formed. Then, the reflective electrode layer 242 is covered with a water-soluble resin layer 262. FIG. 13A is a cross-sectional view at this stage. Note that, for simplification, a cross-sectional structure of the periphery of the reflective electrode layer 242 is illustrated but a photodiode formed over the same substrate is not illustrated in FIG. 13A.

Next, the water-soluble resin layer 262 is fixed to a supporting substrate or the like, and an opening is formed by irradiation of the separation layer with laser light or the like, whereby a layer including the transistor is separated from the substrate 230. FIG. 13B is a cross-sectional view at this stage. As illustrated in FIG. 13B, separation is performed at an interface between the separation layer 260 provided with the substrate 230 and the oxide insulating film 261.

Then, as illustrated in FIG. 13C, a flexible substrate 264 is attached to a surface of the layer including the transistor (a surface which is exposed by the separation) with an adhesive layer 263. As the flexible substrate 264, a plastic film or a thin stainless-steel substrate can be used.

Next, the water-soluble resin layer 262 is removed and an alignment film 244 is formed. Then, a counter substrate 268 including a counter electrode 267 and the flexible substrate 264 are attached to each other with a sealant. Note that before the attachment, an alignment film 266 covering the counter electrode 267 is formed for the counter substrate 268. In the case of using a liquid crystal dropping method, liquid crystal is chopped onto a region surrounded by a sealant of a closed loop and attachment of the pair of substrates is performed under reduced pressure. In this manner, a region surrounded by the pair of substrates and the sealant is filled with a liquid crystal layer 265. When a plastic film with high light-transmitting properties and small retardation is used as the counter substrate 268, a flexible liquid crystal panel can be manufactured.

The above-described process for manufacturing a flexible liquid crystal panel is only one example. Alternatively, for example, a flexible liquid crystal panel may be manufactured in such a manner that glass substrates which are used as the substrate 230 and the counter substrate 268 are thinned by polishing or the like after the transistor is manufactured. In the case of thinning by polishing, both of the substrate 230 and the counter substrate 268 are thinned by polishing after filling with the liquid crystal layer is performed.

FIGS. 14A and 14B illustrate an example of an electronic book reader using a liquid crystal panel.

FIGS. 14A and 14B as an example of an electronic book reader illustrate the case where a supporting portion 4308 is provided at an end portion of a liquid crystal panel 4311. The specific structure of the electronic book reader is described below with reference to FIGS. 14A and 14B. FIG. 14A illustrates the electronic book reader which is horizontally disposed, and FIG. 14B illustrates the electronic book reader which is vertically disposed.

The electronic book reader illustrated in FIGS. 14A and 14B includes the flexible liquid crystal panel 4311 including a display portion 4301, the supporting portion 4308 provided at the end portion of the liquid crystal panel 4311, a scan line driver circuit 4321 a for controlling the display of the display portion 4301, a photo sensor driver circuit 4321 b for controlling a photodiode provided in the display portion 4301, and a signal line driver circuit 4323 for controlling the display of the display portion 4301.

The scan line driver circuit 4321 a and the photo sensor driver circuit 4321 b are provided over a flexible surface of the liquid crystal panel 4311, and the signal line driver circuit 4323 is provided inside the supporting portion 4308.

It is preferable that the supporting portion 4308 be less flexible (more rigid) than at least the liquid crystal panel 4311. For example, a housing forming the supporting portion 4308 can be formed using plastic, metal, or the like which is thicker than the liquid crystal panel 4311. In that case, the electronic book reader can be bent (warped) in a portion other than the supporting portion 4308.

There is no particular limitation on where to provide the supporting portion 4308. For example, the supporting portion 4308 can be provided along the end portion of the liquid crystal panel 4311. For example, as shown in FIGS. 14A and 14B, in the case where the liquid crystal panel 4311 has a rectangular shape, the supporting portion 4308 can be provided along a predetermined side of the liquid crystal panel 4311 (so that the side is fixed). Note that the “rectangular shape” here includes a shape in which a corner of the rectangular is rounded.

Further, when the scan line driver circuit 4321 a, the photo sensor driver circuit 4321 b, and a pixel circuit included in the display portion 4301 are formed over the substrate having flexibility through the same process, the scan line driver circuit 4321 a and the photo sensor driver circuit 4321 b can be bent and a reduction in cost can be achieved.

The pixel circuit included in the display portion 4301 and elements included in the scan line driver circuit 4321 a and the photo sensor driver circuit 4321 b can be formed using thin film transistors or the like. On the other hand, a high-speed operation circuit such as the signal line driver circuit 4323 can be formed using all integrated circuit (IC) formed using a semiconductor substrate such as a silicon substrate or an SOI substrate, and the IC can be provided inside the supporting portion 4308.

The liquid crystal panel 4311 is formed using a flexible substrate, and therefore, data can be input without any problem even when the screen is bent because of touch input with the use of a photodiode. Thus, the operability of the liquid crystal panel 4311 is better than other electronic devices having a touch input system.

Note that an example in which a metal layer is used as the separation layer in this embodiment; however, an embodiment is not limited thereto. A separation method with the use of ablation with laser irradiation, a separation method with the use of an organic resin, or the like can be used.

Embodiment 6

In this embodiment, an example of the structure of a scan line driver circuit of a liquid crystal panel is described with reference to drawings.

FIG. 15 is a block diagram of a driver circuit described in this embodiment. In the case where the driver circuit is used as a gate driver (scan line driver circuit) of VGA, it is necessary to drive 480 gate lines, and therefore, data lines for 9 bits are needed. In this example, an example of 3 bits is described.

A block 701 denotes a circuit that generates a gate signal of a first stage, a block 702 denotes a circuit that generates a gate signal of a second stage, a block 703 denotes a circuit that generates a gate signal of a third stage, a block 704 denotes a circuit that generates a gate signal of a fourth stage, and a block 705 denotes a circuit that generates a gate signal of a fifth stage. In the case of VGA, there are also blocks that generate gate signals of 6th to 480th stages in addition to the above.

Data0 a, Data0 b, Data1 a, Data1 b, Data2 a, and Data2 b denote data lines. Character of any of 0 to 2 which is the second from the last character of a signal name corresponds to 3-bit data. As for the last characters “a” and “b” of the signal names, “b” corresponds to an inverted signal of “a” but is not a completely-inverted signal. Data0 a to Data2 b are set to 0 (low, also referred to as GND) in a period in which data is not input. A method for connecting data lines and blocks in respective stages is as follows: when the first stage represents “001” in binary, since Data0 a or Data0 b corresponds to a lowest bit and a lowest bit of the first stage is “1”; therefore, the block of the first stage is connected to one whose last character is “b”, i.e., Data0 b. In a similar manner, since a bit next to the lowest bit is “0”, the first stage is connected to one whose last character is “a”, i.e., the Data1 a.

FIG. 16 shows a relation among data lines with respect to time. In a period 1 in which the block 701 of the first stage is selected, Data2 a is set to 0 (low), Data1 a is set to 0 (low), and Data0 a is set to 1 (high), which corresponds to “001” that is binary representation of 1. In a period 2 in which the block 702 of the second stage is selected, Data2 a is set to 0 (low), Data1 a is set to 1 (high), and Data0 a is set to 0 (low), which corresponds to “010” that is binary representation of 2. As for the third stages and subsequent stages, data are also determined in the same manner.

In addition, when the Data0 a is 0 (low), Data0 b corresponding thereto is set to 1 (high) inversely, whereas when Data0 a is 1 (high), Data0 b is set to 0 (low). The same relation between Data1 a and Data2 a applies to Data1 b and Data2 b. Note that a period in which both of Data0 a and Data0 b are 0 (low) is inserted between the period 1 in which the block 701 of the first stage is selected and the period 2 in which the block 702 of the second stage is selected and serves as a period in which data is not input.

FIG. 17 is a diagram of a circuit forming the inside of the block 701. The same applies to circuits forming the insides of the block 702, the block 703, the block 704, and the block 705. Also in FIG. 17, an example of 3 bits is described. An n-channel transistor group 802, an n-channel transistor 803, an n-channel transistor 804, and an n-channel transistor group 806 which are illustrated in FIG. 17 are formed over the same substrate as that of a transistor of a display portion, and an oxide semiconductor layer is used as a channel in each of the transistor groups and the transistors.

Data0 in FIG. 15 and Data0 in FIG. 17 correspond to each other, and Data0 is electrically connected to Data0 a or Data0 b of FIG. 15. A node 801 has a function of holding data. Although data may be held by a capacitor, since parasitic capacitance is acceptable, a capacitor for holding data is omitted in this embodiment. When one of data lines Data0 to Data2 is set to 1 (high), one of transistors of the n-channel transistor group 802 is turned on and the node 801 is set to 0 (low). When all of the data lines Data0 to Data2 are 0 (low), the node 801 is kept at 1 (high) and this block is regarded as being selected. Further, in a period in which data is not input, all of the data lines Data0 to Data2 are set to 0 (low) and the whole transistor group 802 is turned off.

In order set the node 801 to 1 (high), a reset signal is set to 1 (high), whereby the transistor 803 that is an n-channel transistor is turned on. Note that even when the transistor 803 is turned on, the node 801 does not always have the same potential as VDD because of the threshold value of the transistor 803, which does not cause a problem. When a period in which the reset signal is 1 (high) does not overlap with a period in which one of the data lines Data0 to Data2 is 1 (high), increase of a current flowing from a power supply VDD to GND can be prevented.

When all the data lines are 0 (low) in a period of data input, that is, when the block is selected, the node 801 is kept at 1 (high) and the transistor 804 that is an n-channel transistor is turned on. Further, when this block is selected, the node 801 is not electrically connected to the power supply VDD and GND. When a write signal is changed from 0 (low) to 1 (high), the potential of the node 801 is raised due to capacitive coupling of a capacitor 805. A circuit of the capacitor 805 is referred to as a bootstrap circuit.

After being raised, the potential of the node 801 is preferably higher than a potential obtained by adding the threshold value of the transistor 804 to the highest potential of the write signal. After the potential of the node 801 is raised, if the potential of the node 801 is higher than the potential obtained by adding the threshold value of the transistor 804 to the highest potential of the write signal, the capacitor 805 is not needed or parasitic capacitance is enough in some cases.

After the potential of the node 801 is raised, in the case where the potential of the node 801 is lower than the potential obtained by adding the threshold value of the transistor 804 to the highest potential of the write signal, there is a possibility that the potential of a node Out is not increased to the highest potential of the write signal and writing to a pixel is not performed in time. When the potential of the node 801 is higher than the potential obtained by adding the threshold value of the transistor 804 to the highest potential of the write signal, the write signal is change from 0 (low) to 1 (high), and the node Out is also changed from 0 (low) to 1 (high). The node Out is connected to a gate line of a pixel. Next, when the write signal is changed from 1 (high) to 0 (low), the potential of the node 801 is lowered due to capacitive coupling of the capacitor 805; however, the potential of the node 801 is approximately equal to the potential of VDD supplied by the reset signal and the transistor 804 is not tuned off. In other words, since the potential of the node 801 is higher than a potential obtained by adding the threshold value of the transistor 804 to the level of 0 (low) of the write signal, the node Out is also 0 (low).

When one of the data lines is 1 (high) in a period of data input, that is, when the block is not selected, the node 801 is 0 (low) and the transistor 804 is turned off.

Even when the write signal is changed from 0 (low) to 1 (high), since the transistor group 802 is turned on, the potential of the node 801 is 0 (low) and the transistor 804 is turned off. Since when the transistor group 802 has a low ability to pass current, the potential of the node 801 is raised due to capacitive coupling of the capacitor 805; therefore, it is necessary that the ability to pass current of the transistor group 802 is determined so that change of the node Out is small.

Next, even when the write signal is changed from the 1 (high) to 0 (low), since the transistor group 802 is turned on, the potential of the node 801 is 0 (low) and the transistor 804 is turned off. When the transistor group 802 has a low ability to pass current, the potential of the node 801 is lowered due to capacitive coupling of the capacitor 805; therefore, it is necessary that the ability to pass current of the transistor group 802 is determined so that change of the node Out is small.

In the case where the transistor group 806 is not provided, when one of the data lines is 1 (high) in a period of data input, that is, when this block is not selected, the node Out is not electrically connected to the power supply and may be influenced by a video signal. The potential of the node Out is also changed by capacitive coupling between a source electrode and a drain electrode of the transistor 804. Therefore, in a period in which the transistor 804 is turned off, the node Out is preferably fixed at 0 (low).

FIG. 18 is diagram of time and potentials of nodes. Operation as for the circuit diagram of FIG. 17 is described using FIG. 18.

In a period 901, the node 801 is set to 1 (high) by the reset signal. In the period 901 although the transistor 804 is turned on, the write signal is 0 (low) and therefore, the potential of the node Out is also 0 (low). A next period 902 is a period until the beginning of data input, in which the reset signal is set to 0 (low). It is preferable that increase of current flowing from the power supply VDD to GND be prevented by providing the period 902.

A next period 903 is needed for determining the potential of the node 801 by input data, and the case where all of the data lines Data0 to Data2 are set to 0 (low) is described here. In the beginning of a next period 904, the write signal is set to 1 (high) and the potential of the node 801 is raised. In the period 904, the potential of the node Out is also set to 1 (high).

In a next period 905, the write signal is set to 0 (low). Although the potential of the node 801 is also lowered, since the transistor 804 is turned on, the potential of the node Out is 0 (low). In a next period 906, the period of data input is terminated, and in a next period 907, the reset signal is set to 1 (high) again and the potential of the node 801 is set to 1 (high). The above is one horizontal period, and after that, the horizontal period is repeated. A period which is provided in place of the period 903 and in which one of the data lines Data0 to Data2 is set to 1 (high) is denoted by a period 908. In the period 908, when one of the data lines Data0 to Data2 is set to 1 (high), the potential of the node 801 is set to 0 (low). If the period 908 is short, and the write signal is set to 1 (high) in a next period 909 before the potential of the node 801 is sufficiently lowered by turning off the transistor 804, there is possibility that the potential of the node Out cannot be fixed only by the transistor group 806 and is increased temporarily. If the transistor 804 is turned off in the period 908, the node Out is kept at 0 (low) even when the write signal is set to (high) in the next period 909. Note that the potential of the node Out tends to increase due to parasitic capacitance between the source electrode and the drain electrode of the transistor 804 even when the transistor 804 is turned off. It is necessary that the potential of the node Out is adjusted by the transistor group 806 in order not to turning on the pixel transistor. A load of a gate line is large, and therefore, the parasitic capacitance between the source electrode and the drain electrode does not cause a large problem.

In this embodiment. “1 (high)” is described as the potential of VDD and “0 (low)” is described as the potential of GND. When the highest potential of the write signal is lower than VDD, a bootstrap circuit is not needed, but it is not preferable that two kinds of power supplies are provided on the outside because cost is increased. It is possible to perform operation using one power supply in this embodiment.

Note that the structure of the driver circuit of the display device described in this embodiment can be implemented in free combination with any of structures in other embodiments in this specification.

In the structure of the driver circuit of the display device described in this embodiment, when the write signal and each of the data lines are set to 0 (low) and the reset signal is set to 1 (high), gate lines of all of the pixels are set at 0 (low). Unlike a case where a shift register circuit is used for a driver circuit of a display portion, a gate line of a pixel in an arbitrary row can be set to 1 (high) in an arbitrary order.

This embodiment is one example of a decoder circuit included in a driver circuit, and therefore, an embodiment is not limited to the structure of the driver circuit of the display device described in this embodiment.

This application is based on Japanese Patent Application serial No. 2010-050947 filed in Japan Patent Office on Mar. 8, 2010, the entire contents of which are hereby incorporated by reference. 

1. An electronic device comprising: a display portion having a touch-input function; and a first transistor electrically connected to a first reflective electrode, a second transistor electrically connected to a second reflective electrode, and a photo sensor, over a flexible substrate, wherein the photo sensor comprises: a photodiode; a third transistor including a gate signal line electrically connected to the photodiode; and a fourth transistor, wherein one of a source and a drain of the third transistor is electrically connected to a photo sensor reference signal line, the other of the source and the drain of the third transistor is electrically connected to one of a source and a drain of the fourth transistor, and the other of the source and the drain of the fourth transistor is electrically connected to a photo sensor output signal line, wherein an oxide semiconductor layer of the fourth transistor overlaps with the first reflective electrode, and wherein the photo sensor reference signal line overlaps with the second reflective electrode.
 2. The electronic device according to claim 1, wherein an oxide semiconductor layer of the third transistor overlaps with a reading signal line with a gate insulating layer provided therebetween, and wherein the reading line overlaps with the first reflective electrode.
 3. The electronic device according to claim 1, wherein one of the source and the drain of the fourth transistor overlaps with the first reflective electrode and the other of the source and the drain of the fourth transistor overlaps with the second reflective electrode.
 4. An electronic device comprising: a display portion having a touch-input function; and a first transistor electrically connected to a reflective electrode, and a photo sensor over a flexible substrate, wherein the photo sensor comprises: a photodiode; a second transistor including a gate signal line electrically connected to the photodiode; and a third transistor, and wherein one of a source and a drain of the second transistor is electrically connected to a photo sensor reference signal line, the other of the source and the drain of the second transistor is electrically connected to one of a source and a drain of the third transistor, and the other of the source and the drain of the third transistor is electrically connected to a photo sensor output signal line.
 5. The electronic device according to claim 4, wherein an oxide semiconductor layer of the second transistor overlaps with a reading signal line with a gate insulating layer provided therebetween, and wherein the reading signal line overlaps with the reflective electrode. 